I think the area South/South West of Wetlands could do with a little redecoration. It is mostly open ground. My suggestion is there could be a maze of trenches in the ground. This would be an ideal location to place Caustic traps and perimeter fences.
This wouldn’t be underground. The bunkers I speak of would be above ground with only a door or two and long windows to shoot out of. Kind of like the bunkers they had at the beaches of Normandy in WW2.